Critical to the performance and quality of material processing with a fibre laser is optimisation of the focus. A typical processing arrangement has a lens with variable focus position, a gas assist delivered via a nozzle and the workpiece itself usually mounted on some form of controllable translation stage. Before the advent of high powered continuous wave (cw) fibre lasers this form of processing would be typically carried out with a high powered solid state laser of 200 W to 4 kW. Lasers of this type would be relatively low brightness M2>10 and be delivered in a large mode area multimode fibre, eg 400 μm 0.1 measured Numerical Aperture (NA) equivalent to an M2 of 60. Typically used with one to one imaging a laser of this type would produce a focus of 400 μm with a confocal parameter (ie depth of field) of +/−2 mm. Comparing to a high brightness fibre laser source the working spot size is at least an order of magnitude larger than that of a fibre laser but the confocal parameter is about 20 times larger consequently focus is less critical. The parameter M2 is defined further later on in this document.
Single mode fibre lasers are now capable of up to 10 KW in a single mode. The beam quality from a single mode fibre laser is a near perfect M2 of 1 consequently the output of a fibre laser can be focussed to a much smaller spot than conventional solid state lasers. This decreased spot size, typically as small as 10 μm diameter, creates similar intensities as conventional lasers but at a much lower power. This enables processing of for like materials at much lower powers compared with conventional lasers. It is advantageous commercially to process with as low a power as possible as there is an approximate linear relationship between the price of a fibre laser and its power. Processing with this low power requires that the focus position relative to the workpiece and gas delivery nozzle is finely optimised. A beam with an M2 of 1 and diameter 10 μm will have a confocal parameter of +/−75 μm illustrating the potential tolerance requirements of processing with a beam of this nature.